1. Field of the Invention
This invention generally relates to semiconductors and fabricating methods thereof, and more particularly, to a photomask and a focus monitoring in the photolithography process and to a technique for fabricating a semiconductor device by monitoring a slant of an image plane of projection exposure.
2. Description of the Related Art
With the miniaturization of the pattern used for fabricating semiconductor devices, the error margin becomes smaller for exposure conditions such as focal depth (focus) or light amount adjustment in the photolithography process for pattern transfer. Hence, there is a need for a method of monitoring the exposure conditions with high accuracy.
FIGS. 1A through 1C are cross-sectional views qualitatively illustrating the relationship between an exposure amount and a pattern width. FIG. 1A shows an appropriate exposure amount. FIG. 1B shows an excessive (over) exposure amount. FIG. 1C shows an insufficient (under) exposure amount. FIGS. 1A through 1C respectively illustrate resist patterns (11a through 11c) obtained by exposure. Here, a reference numeral 10 represents a substrate on which the resist is applied. As shown in FIGS. 1A through 1C, a narrower pattern is obtained in the excessive exposure amount (FIG. 1B) than that of the appropriate exposure amount (FIG. 1A). On the contrary, a wider pattern is obtained in the insufficient exposure amount (FIG. 1C) than that of the appropriate exposure amount (FIG. 1A). It is common to monitor the exposure amount by monitoring the pattern width, thereby giving feedback of the monitoring result to the control for the exposure amount in order to avoid a change in the device characteristics or a defect caused by variation in the pattern width.
FIGS. 2A through 2C are schematic views qualitatively illustrating the relationship between focus (misalignment) and a cross-sectional shape. FIG. 2A shows an appropriate focus amount (best focus). FIG. 2B shows an excessive focus amount (over focus). FIG. 2C shows an insufficient focus amount (under focus). FIGS. 2A through 2C respectively illustrate resist patterns (21a through 21c) obtained by exposure. As shown in FIG. 2A, a pattern of a trapezoid having an upper base shorter than a lower base is obtainable in the best focus. On the contrary, a pattern of a (reverse) trapezoid having the upper base longer than the lower base is obtained in the over focus case as shown in FIG. 2B. Also, in the under focus case as shown in FIG. 2C, the upper base becomes “rounded”, resulting in an upwardly curved pattern. In this manner, variation of the focus amount from the appropriate amount largely affects the pattern shape to be formed. Therefore, it is extremely important to control the focus amount. However, as shown in FIGS. 2A through 2C, even if the focus amounts vary from the appropriate value, a defect in the pattern can be seen only on the lower base (bottom) or in an upper portion. This is rarely recognized as a defect in the line width of the pattern if the pattern is monitored from above the wafer with the use of the method of pattern recognition. This is the reason why a monitoring method is needeed to effectively monitor misalignment from the appropriate focus.
FIG. 3 is a flowchart explaining an overview of an example of a conventional focus monitoring method. In the conventional focus monitoring method, a pattern most susceptible to the influence of the focusing condition is selected from among the patterns formed on the semiconductor device during the fabrication process, and the line width of the pattern is measured. Specifically, first, the exposure process is performed on the product (step S301). The pattern most susceptible to the influence of the focusing condition is selected from among the patterns obtained and the line width thereof is measured (step S302).
The line width of the pattern is compared with a design value (step S303). If the line width of the pattern is equal to the design value (step S303: Yes), the product is forwarded to the next stage of the process (step S309). In contrast, if the line width of the pattern is different from the design value (step S303: No), it is determined that the exposure amount or the focus amount is not appropriate. A new exposing condition is set again with parameters for the exposure amount and the focus value (step S304). Then, the appropriate exposure amount and focus value are obtained (step S305), and it is judged whether or not the focusing condition in step S301 is different from the appropriate focus (step S306).
If the focusing condition is judged inappropriate (step S306: Yes), the cause of the misalignment in focus is investigated, specified, and modified to set the appropriate focusing condition (step S307), and then the product is forwarded to the next stage of the process (step S309). In contrast, if the focusing condition is judged appropriate (step S306: No), it is considered that the exposure amount is different from the appropriate value and the exposure value is corrected (step S308), and then the product is forwarded to the next stage of the process (step S309).
In addition to the monitoring method of the above-described exposing condition with a high degree of accuracy, there is another extremely important technique in semiconductor fabrication which is the method of monitoring the slant of an image plane at the time of projection exposure. It is a precondition for ensuring high yields that the whole surface of the wafer is properly exposed for mounting a number of chips on a wafer having a large diameter. Originally, the wafer is “distorted” such as warp or bow caused by wafer production and, in addition, may newly be distorted by thermal processing during the fabrication process. This is a problem because it is not always easy to set the wafer to be projected and exposed flat relative to a lens (optical system) mounted on a photolithography machine.
Conventionally, there has been employed a method of monitoring the slant of the image plane to be projected and exposed with the use of a test wafer (a method for measuring the plane to adjust the leveling). For example, “ink baking” of various marks is a known method. This “ink baking” is a method for exposing an extremely small area in an extremely small step while the focus is being changed little by little. Specifically, the pattern for monitoring arranged on the reticle is baked in the extremely small area in the extremely small step while the focus is being changed. The extremely small area is baked so as, as much as possible, to not be affected by the curvature of the image plane (curvature caused resulting from, for example, wafer curvature, nonuniform resist application, and the distribution in development) in an effective range for baking.
FIGS. 4A through 4C are views illustrating examples of the patterns used for monitoring the slant of the image plane and dependency of the line width of the pattern on the focus amount. Here, in the drawings, the reference numeral 10 represents the substrate and reference numerals 41 and 42 represent patterned resists. If the patterns, as shown in FIGS. 4A and 4B as top views, are adjacently provided to the wafer surface and baked by changing the focus amount by a certain amount every time, the line widths (such as L1 and L2) vary according to the focus amount. This is because the line width of the pattern has the maximum value when exposed in the best focusing condition (zero in the focus misalignment) and the line width becomes narrower during the under focus or over focus condition, once the best focusing condition is not met. Therefore, the best focus point corresponds to a peak in the correlation curve of the focus amount and the line width of the pattern, which is obtained by changing the focus amount by a certain amount every time. In other words, it is possible to know the best focus point by exposing multiple marks with different focuses and measuring the line widths of the pattern obtained.
FIG. 5 is a view illustrating an example of monitoring the slant of the image plane with the use of the dependency of the line width of the pattern on the focus amount. Multiple chip patterns are baked in an exposure region 50 to be exposed by one shot (a single exposure step). In this drawing, the exposure region 50 for one shot includes the slant of the image plane. Assuming that points A and B are in the best focusing condition from among four corners of the exposure region, yet on the other hand, points C and D are in the over focusing condition.
The conventional focus monitoring method is configured to compare the pattern width and the design value, as described above. Therefore, it is difficult to know whether the exposure amount or the focus amount is different from the appropriate value, since both affect the variation in the pattern width. This leads to the problem that a complicated procedure is necessary for finding the appropriate focusing condition by using the wafer having the parameters of the exposure amount and the focus amount. That is to say, it is difficult to sense the “difference” from the appropriate value immediately, even if the focusing condition is not equal to the appropriate value in the conventional focus monitoring method. This complicates the fabrication process of the semiconductor device and degrades the throughput.
In addition, in the conventional method of monitoring the slant of the image plane, a large part of the shots result in multi-exposure, because the marks to be included in the periphery or inside of the exposure region are exposed in an extremely small area. However, the afore-mentioned multi-exposure is not performed on the device region of the wafer of the product, and the mark used for checking the leveling cannot be baked in the product reticle. This is the reason why the establishment of a monitoring method with the use of the product reticle is needed so that the flatness (the slant of the image plane) of the photolithography machine used for exposing the surface of the wafer in an actual product to be affected by a factor such as distortion of the wafer or slant of the stage can be monitored relative to the lens system (optical system). That is to say, a monitoring method is needed to monitor the slant of the image plane of an actual product during the fabrication process.